Audio signals stored in digital form have been in use for decades; however, distribution of such digital audio signals has generally been limited to physical distribution of tangible storage media in which the digital audio signals are encoded. Examples include compact discs (CDs) and digital audio tape (DAT) which store audio signals representing, for example, pre-recorded music, spoken word recordings, and sound effects. Recently, wide area computer networks such as the Internet have experienced tremendous growth in use and popularity. Accordingly, direct delivery of digital audio signals through such a wide area network has become an alternative to, and threatens to replace, physical delivery of tangible storage media as the primary delivery mode of digital audio signals.
Many digital audio signal filtering systems are currently available. Many such systems are used, for example, in producing a “master” signal in which various component signals, e.g., each from a separate musical instrument, are filtered and mixed such that the resulting master signal represents the artistic creation of an artist or collection of collaborating artists. This master signal is what is typically fixed in the tangible storage media which is physically distributed to the consuming public.
In direct delivery of digital audio signals through wide-area computer networks, the master signal can be sent directly to the computer system of a consumer. The master signal can be played directly from the consumer's computer system through a sound card and attached loudspeakers or can be stored on a tangible storage medium, e.g., writeable CD-ROM, for playback using conventional CD players and analog stereo equipment. Since the master signal is digital and is the same master signal which would traditionally be stored in tangible storage media by the producer, the master signal received by the consumer through the wide-area computer network is of the same quality as the master signal physically distributed on tangible storage media.
Sometimes, samples of the master signal are made available to the consumer through the computer network for preview purposes. Such samples are frequently streamed, i.e., delivered to a client computer system while the client computer system decodes and plays the received digital audio signals in real time. Because of variations in bandwidth with which various client computer systems are attached to computer networks such as the Internet, such samples are frequently delivered through low bandwidth communications media which are incapable of real-time delivery of such digital audio signals in a native, un-compressed form. Accordingly, the digital audio signal is generally compressed and encoded to reduce the amount of data required to represent the digital audio signal. The digital audio signal can be transmitted through computer network media in less time, requiring less bandwidth, than would ordinarily be required to transmit the digital audio signal in its native, un-encoded form. However, compression of the digital audio signal usually results in loss of detail of the digital audio signal such that sound quality of a received, decoded digital audio signal is typically degraded from the sound quality of the original digital audio signal prior to encoding and delivery through a computer network.
To mitigate the loss of signal quality as a result of such compression or to reduce some of the annoying effects of such compression, some sound engineers apply filters to a digital audio signal to enhance the result of compressing and encoding the digital audio signal. For example, in certain circumstances, emphasizing certain frequencies while de-emphasizing other frequencies of a digital audio signal prior to compression and encoding produces an encoded digital audio signal which has a more pleasant sound when decoded and played back relative to the sound of playback of a digital audio signal which is not filtered prior to such encoding. However, finding a particularly good combination of filters and encoders for a particular digital audio signal typically requires application of different filters from different suppliers and iterative application of such filters with various encoders to find an optimal combination. Furthermore, once a good combination of filters and encoders is determined for a particular digital audio signal, the combination is often not the best combination for a different digital audio signal and tile entire empirical selection of a good combination of filters and encoders must generally be repeated for the different digital audio signal.
In addition, when distributing digital audio signals through a wide area computer network, it is sometimes desirable to deliver the digital audio signal within a particular amount of time. Such is desirable when streaming digital audio signals for real time playback. In such circumstances, the encoding of the digital audio signal should be tailored to the particular bandwidth of the network communications media connecting a particular recipient computer system with a source computer system within the computer network. In heterogeneous computer networks, various recipient computer systems can be connected with widely different bandwidths. For example, computer systems connected to the Internet are connected through network media ranging from 14.4 k modems to dedicated T1 connections which have many times the bandwidth of 14.4 k modems. Accordingly, encoding a digital audio signal for one recipient computer system having a particular bandwidth produces an encoded audio signal which is unacceptable for other recipient computer systems. For example, encoding a digital audio signal for real time delivery through a 14.4 k modem produces an encoded audio signal in which signal quality is unnecessarily sacrificed if the encoded signal is delivered to a recipient computer system connected to the source computer system through a dedicated T1 connection. Conversely, encoding a digital audio signal for real time delivery through a dedicated T1 connection produces an encoded audio signal which exceeds the available real-time delivery bandwidth of a recipient computer system connected to the source computer system through a 14.4 k modem.
Further exacerbating the problem is that application of a combination of filters prior to encoding to produce a reasonably good quality encoded audio signal when encoded for a particular delivery bandwidth can produce an encoded audio signal of unacceptable quality when the same combination of filters is applied prior to encoding the digital audio signal for a different delivery bandwidth. Accordingly, a new combination of filters must be empirically determined for each delivery bandwidth for which a digital audio signal is to be encoded. Therefore, the amount of experimentation with various filters and encoders to deliver reasonably high quality signals to recipient computer systems connected through media of differing bandwidths can be overwhelming.
What is needed is a digital audio signal filtering and encoding system which significantly simplifies the processing of digital audio signals for distribution through heterogeneous computer networks through different delivery bandwidths.